The transition of the non-tumorigenic C3H10T1/2 cells from a state of controlled growth to the neoplastic state was achieved under conditions of in vivo selection pressure by subcutaneous implantation of these cells attached to 1x5x10 mm plastic plates. Cell lines from four different C3H10T1/2 tumors so formed explanted back in vitro showed coordinate loss of density inhibition of proliferation, anchorage dependence, and high serum growth requirement. This was further evidence that these changes were an integral part of the process of neoplastic progression in vivo. The degree of loss of density, anchorage, and serum regulation by the tumor cell lines in the first five passages tended to correlate the degree of their growth aggressiveness in vivo. Plasminogen activator activity, though present, did not follow this general correlation with degree of tumor growth aggressiveness. A useful tumor growth aggressiveness index, incorporating both parameters of tumor incidence and latent period, was formulated. Non-tumorigenic cells were observed to be blocked in portion of cell-cycle between metaphase & early G1, when grown on a non-adhesive substrate like Teflon. BIBLIOGRAPHIC REFERENCE: Paranjpe, M.S., Boone, C.W., and Takeichi, N.: Specific paralysis of anti-tumor cellular immune response produced by growing tumors studied with a radioisotopic footpad assay. Annals of the New York Academy of Sciences. 276: 254-259, 1976.